1. Field of the Invention
The present invention relates to a liquid crystal display device using a liquid crystal material having ferroelectricity or antiferroelectricity superior in a high-speed response.
2. Prior Art
The liquid crystal display device performs display by applying an electric field to the crystal material from the exterior so that the optical anisotropy of the material is changed to thereby convert an electric signal into a light signal.
FIG. 1 shows a structure of a general liquid crystal display device. The liquid crystal display device includes electrodes 103 and 104 formed on substrates 101 and 102 for driving a liquid crystal material. The electrodes 103 and 104 are disposed in such a manner that the respective electrodes 1 03 and 104 on the substrates 101 and 102 face each other. A liquid crystal material 105 is interposed between the substrates 101 and 102. The liquid crystal devices which utilize birefringence of the liquid crystal material have been widely known. In order to effectively utilize the birefringence of the liquid crystal material, one side of the respective substrates which is brought in contact with the liquid crystal material 105 is subjected to some orienting process 106 so that an optical axis of the liquid crystal material 105 is oriented in an intended direction.
Among the above-mentioned liquid crystal display devices, ones using the liquid crystal material having ferroelectricity or antiferroelectricity provide such an excellent characteristic that a response speed is approximately 1,000 times as high as that of the TN type or STN type liquid crystal display device.
As described above, in the liquid crystal device using the liquid crystal material having the ferroelectricity or antiferroelectricity, the surfaces of two substrates, which interpose the liquid crystal material therebetween and is to be in contact with the liquid crystal material are subjected to orienting process, and in the case of using the liquid crystal material, a uniaxial orientation is given to the liquid crystal material. As the orienting processes, there have been known a method of obliquely depositing SiO.sub.2 on two substrates interposing a liquid crystal material therebetween, a rubbing method of forming dielectric thin films on the surfaces of two substrates at the side of electrodes formed thereon as oriented films and of rubbing the surfaces of the dielectric films, and a method of applying an electric field or a magnetic field to a liquid crystal material from the exterior. Among these orienting processes, the rubbing method is superior in view of industry.
In the rubbing method, in general, an orientation film of 100 to 1000 .ANG. is formed on at least one side of two substrates of the liquid crystal display device, which is in contact with the liquid crystal material, and the surface of the orientation film is subjected to the rubbing process, that is, the surface of the film is rubbed with cloth made of cotton, nylon or the like. The orientation films are made of organic component such as polyvinyl alcohol, nylon, polyimide or the like, or inorganic component such as silicon oxide.
When the liquid crystal has been optically uniaxially oriented by the rubbing process, the molecules of the liquid crystal are not arranged in parallel with the substrates interposing the liquid crystal therebetween, but is obliquely oriented with a certain oblique angle. In general, this angle is called a pretilt angle. Conventionally, there has been considered that it is good to set the pretilt angle to 5.degree. or more in view of various points.
However, the liquid crystal using the liquid crystal material with ferroelectricity or antiferroelectricity has the following problems.
When the liquid crystal display device manufactured by orienting the liquid crystal material with ferroelectricity or antiferroelectricity is actuated by means of a square wave, there may occur a portion within a pixel where a light is leaked while displaying the dark state. Such a light leakage causes the contrast of the liquid crystal display device to be lowered.
In the liquid crystal display device of this type, in the case where the orientation state of the liquid crystal material is observed under the crossed Nicol through a polarization microscope, it is recognized that while the greater parts of the liquid crystal material are optically uniaxially oriented, however, defects in a line state or zigzag state occur partly.
Further, if the display mode is switched between the dark state and the bright state, it takes much time to obtain a given quantity of transmitted light after changing between the dark state and the bright state. Also, if any state is maintained as it is, there is a case where the quantity of transmitted light is changed. For example, there has been found such a phenomenon that the quantity of transmitted light of a pixel is gradually increased in the dark state.
Since the liquid crystal display device like this is unstable in the optical characteristics, it is not suitable for simple matrix drive for driving the display device by means of a pulse waveform and active matrix drive.
FIG. 2 shows a current-to-voltage characteristic of such a liquid crystal display device. Shown in the figure is a case where the liquid crystal material with ferroelectricity is used. The current-to-voltage characteristic has been measured by connecting a resistor of 100 k.OMEGA. in serial to the liquid crystal display device, and by measuring a voltage drop of the resistor through an oscilloscope when applying a chopping wave of the 5 Hz frequency and the .+-.30 V voltage to the element. The current value in the figure represents a value resulting from dividing a voltage read by the oscilloscope by the above-mentioned resistance value. As shown in FIG. 2, the current component is classified into three parts, that is, a component 201 acting as a capacitor disposed between the electrodes of the liquid crystal display device, a current component 202 flowing when the spontaneous polarization of the liquid crystal material is inverted with the direction of the electric field being changed, and the other current component 203. The third current component will be hereinafter represented by a 2nd peak current.
As the second peak current is large, the initial transmissivity is not held and the optical characteristics become unstable. Therefore, the reduction of the 2nd peak current has been required.